It all depends when! I did ONC in 1971. It was equivalent to 3 A levels and I went to university to do an honours degree.
At that time the HNC with 3rd year endorsement was equivalent to a pass/bachelors degree.
Cant say how it stacks up now.

Firstly, you have a National Certificate not an ONC. The latter were dumped about 1970. They were difficult to pass, because there were examinations, but were quite valuable.
A National Certificate is comparable to A levels inasmuch as it is an entry qualification into Higher Education. With high UCAS points (Distinctions and Credits) you could get to University.
It is also a NVQ 3 qualification but more academic than C&G courses.